[common; from the novel Stranger in a Strange
Land, by Robert A. Heinlein, where it is a Martian word meaning
literally ‘to drink’ and metaphorically ‘to be one
with’] The emphatic form is grok in
fullness.

1. To understand. Connotes intimate and exhaustive knowledge. When
you claim to ‘grok’ some knowledge or technique, you are
asserting that you have not merely learned it in a detached instrumental
way but that it has become part of you, part of your identity. For
example, to say that you “know” LISP is
simply to assert that you can code in it if necessary — but to say
you “grok” LISP is to claim that you have deeply entered the
world-view and spirit of the language, with the implication that it has
transformed your view of programming. Contrast zen,
which is similar supernal understanding experienced as a single brief
flash. See also glark.

2. Used of programs, may connote merely sufficient understanding.
“Almost all C compilers grok the void
type these days.”